New Thermal Neutron Scattering Kernels for Light and Heavy Water Based on Molecular Dynamics Simulations

Calculations of moderators require scattering kernels that are accurate in both the energetics and angular distribution of outgo- ing neutrons. Currently, scattering cross sections available in ENDF format to be used in Monte Carlo and deterministic calculations rely on water models that do not utilize the up-to-date knowledge of its structure and dynamics.In this work we present a new model to compute the scattering kernels for light and heavy water based on molecular dynamics simulations. Using the molecular dynamics program GROMACS and a flexible TIP4P model we obtained the frequency spectra of hydrogen, deuterium and oxygen, bound in light and heavy water. With those spectra we computed the scattering laws using the LEAPR module of NJOY, which uses the incoherent approximation. Scattering laws for D and O in D2O are corrected using the Skld approximation and partial structure factors obtained by Soper.Using those kernels we computed double differential and integral cross sections over a wide range of neutron energies (10−5 1 eV). These results represent an improvement over existing models when compared with experimental values, especially for low incident neutron energies.